Affinage

SMCR8

Guanine nucleotide exchange protein SMCR8 · UniProt Q8TEV9

Length
937 aa
Mass
105.0 kDa
Annotated
2026-04-28
17 papers in source corpus 15 papers cited in narrative 15 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

SMCR8 is the catalytic GAP subunit of the trimeric C9orf72–SMCR8–WDR41 complex, functioning as a central regulator of Rab GTPase signaling that integrates autophagy, lysosomal homeostasis, ciliogenesis, and immune tolerance. Within the complex, SMCR8 bridges C9orf72 and WDR41, contributing an arginine-finger residue (Arg147) that stimulates GTP hydrolysis on RAB8A and RAB11A, while C9orf72 serves as the Rab-binding subunit; this GAP activity suppresses primary ciliogenesis and Hedgehog signaling, promotes autolysosome acidification and lysosomal exocytosis, and enables ESCRT-dependent lysosomal repair (PMID:32303654, PMID:38064514, PMID:34297726). SMCR8 associates with the ULK1/FIP200 autophagy initiation complex through a coiled-coil region in its uDENN domain, and loss of SMCR8 impairs autophagic flux, causes mTORC1/AKT overactivation through aberrant MTOR accumulation, dysregulates endosomal TLR signaling, and produces autoimmunity and neurodegeneration phenotypes in mice (PMID:27193190, PMID:31847700, PMID:30442666, PMID:31625563). SMCR8 stability depends on C9orf72 binding and is negatively regulated by CRL2^FEM1B-mediated ubiquitylation of its C-terminal degron (PMID:29950492, PMID:33892462).

Mechanistic history

Synthesis pass · year-by-year structured walk · 10 steps
  1. 2016 High

    Establishing that C9orf72, SMCR8, and WDR41 form a stable trimeric complex linked to autophagy initiation resolved the long-standing question of how C9orf72 exerts its cellular function, placing SMCR8 as a core effector rather than a bystander.

    Evidence Reciprocal co-immunoprecipitation, pulldown assays, and CRISPR knockout mice with autophagy phenotyping across multiple independent labs

    PMID:27193190 PMID:27559131 PMID:27617292

    Open questions at the time
    • Whether the complex acts as a GEF or GAP remained debated (GEF for RAB39B was initially proposed)
    • The structural architecture of the trimeric complex was unknown
    • Mechanism linking the complex to ULK1 activation was not defined
  2. 2016 High

    Demonstrating that SMCR8/C9orf72 loss impairs mTORC1 signaling responses and causes lysosomal swelling established the complex as a nutrient-responsive regulator at the lysosome, not solely an autophagy initiator.

    Evidence CRISPR knockout cells, live-cell lysosomal imaging, mTORC1 signaling assays under amino acid starvation

    PMID:27559131

    Open questions at the time
    • Whether the lysosomal phenotype was a direct or indirect consequence of autophagy impairment was unclear
    • The mechanism of mTORC1 deregulation was not resolved
  3. 2018 High

    Showing that Smcr8 knockout mice develop autoimmunity with increased lysosomal exocytosis and that the splenomegaly is rescued by endosomal TLR ablation identified a specific immune-regulatory mechanism: the complex restricts endosomal TLR signaling duration through lysosomal/phagosomal maturation.

    Evidence Smcr8 KO mice, genetic epistasis with TLR3/7/9 triple knockout, surface LAMP1 and cytokine assays, phagosome maturation kinetics

    PMID:29950492 PMID:30442666

    Open questions at the time
    • Whether the TLR phenotype involves the GAP activity or a GAP-independent scaffolding role was unknown
    • Direct measurement of TLR–ligand contact duration was not provided
  4. 2019 High

    Establishing that SMCR8 loss causes mTORC1/AKT overactivation via defective autolysosome acidification and aberrant MTOR protein accumulation, rescuable by rapamycin, placed autolysosome dysfunction upstream of the signaling phenotype.

    Evidence Double KO mice, lysosomal pH assays, rapamycin rescue of splenomegaly and macrophage dysfunction

    PMID:30696333 PMID:31847700

    Open questions at the time
    • How the complex mechanistically controls lysosomal acidification (e.g., V-ATPase recruitment) was not determined
    • Contribution of individual Rab substrates to this phenotype was unclear
  5. 2019 Medium

    Demonstrating that Smcr8 deficiency impairs axonal autophagy-lysosomal transport and causes motor neuron degeneration, exacerbated by C9orf72 haploinsufficiency, linked the complex to neurodegeneration-relevant axonal biology.

    Evidence Smcr8 KO and Smcr8+/−;C9orf72−/− mice, axonal transport assays, motor behavior testing

    PMID:31625563

    Open questions at the time
    • Whether motor neuron phenotypes arise from autophagy defects, lysosomal defects, or both was not dissected
    • Single lab study; independent replication in a different mouse strain not performed
    • No direct connection to ALS/FTD patient mutations in SMCR8 itself
  6. 2020 High

    The 3.2 Å cryo-EM structure resolved SMCR8 as the bridging and catalytic subunit, identified Arg147 as the arginine-finger essential for GAP activity toward RAB8A and RAB11A, and definitively settled the GEF-versus-GAP debate in favor of GAP function.

    Evidence Cryo-EM at 3.2 Å, in vitro GTPase GAP assays with Arg147 point mutations

    PMID:32303654

    Open questions at the time
    • Structure of the complex bound to a Rab substrate was not determined
    • How WDR41 contributes to activity beyond structural scaffolding was unclear
  7. 2021 High

    Discovery that CRL2^FEM1B targets the SMCR8 C-degron for ubiquitin-dependent turnover, validated by crystal structure, revealed a dedicated protein quality/quantity control pathway for the complex.

    Evidence Crystal structure of FEM1B–SMCR8 C-degron complex, biochemical binding assays

    PMID:33892462

    Open questions at the time
    • Physiological conditions that modulate FEM1B-mediated SMCR8 degradation were not identified
    • Whether the degron is masked by C9orf72 binding was not tested
  8. 2021 High

    A second cryo-EM structure mapped two dimerization interfaces and identified a coiled-coil in the SMCR8 uDENN domain as the starvation-responsive FIP200 interaction platform, mechanistically connecting GAP function to autophagy initiation.

    Evidence Cryo-EM at 3.8 Å, deletion mutagenesis of SMCR8 coiled-coil, co-IP under starvation

    PMID:34297726

    Open questions at the time
    • Whether FIP200 binding and GAP activity are coupled or independent outputs was not resolved
    • The functional significance of the dimer-of-trimers assembly remains unclear
  9. 2023 High

    Assigning C9orf72 as the RAB8A-binding subunit and SMCR8 as the catalytic GAP subunit, and showing this activity suppresses primary ciliogenesis and Hedgehog signaling, extended the complex's functions beyond autophagy/lysosomes to a developmental signaling pathway.

    Evidence In vitro GAP assays with separated subunits, cilia length measurements in KO mice across brain, kidney, and spleen, Hedgehog reporter assays

    PMID:38064514 PMID:38293807

    Open questions at the time
    • Whether ciliogenesis and autophagy functions share the same RAB8A pool or are spatially segregated was not determined
    • Hedgehog phenotypes in human disease context were not examined
  10. 2025 Medium

    Identification of a RAB8A-ESCRT-dependent lysosomal repair pathway controlled by the C9orf72/SMCR8 GAP complex in microglia revealed that loss of GAP activity causes RAB8A hyperactivation/hyperphosphorylation and ESCRT misrecruitment, providing a new mechanism for lysosomal dysfunction in neuroinflammation.

    Evidence (preprint) C9orf72/SMCR8 KO mice, LLOMe-induced lysosomal damage, galectin-3 and ESCRT recruitment assays, phospho-RAB8A imaging

    PMID:bio_10.1101_2025.08.22.671707

    Open questions at the time
    • Preprint not yet peer-reviewed
    • Whether the lysosomal repair mechanism operates in neurons as well as microglia was not tested
    • Identity of the RAB8A kinase responsible for hyperphosphorylation was not determined

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key unresolved questions include: how the complex coordinates its multiple downstream outputs (autophagy initiation, lysosomal acidification, ciliogenesis, lysosomal repair) through a common GAP activity; whether additional Rab substrates exist beyond RAB8A, RAB11A, and RAB39B; and what upstream signals regulate complex assembly, dimerization, and FEM1B-mediated turnover in physiological and disease contexts.
  • No substrate-bound structure of the GAP complex exists
  • Regulation of complex dimerization and its functional significance are unresolved
  • No direct genetic link between SMCR8 mutations and human disease has been established

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0003924 GTPase activity 4 GO:0098772 molecular function regulator activity 3
Localization
GO:0005764 lysosome 4 GO:0005829 cytosol 2 GO:0005929 cilium 2
Pathway
R-HSA-9612973 Autophagy 6 R-HSA-162582 Signal Transduction 4 R-HSA-1852241 Organelle biogenesis and maintenance 3 R-HSA-168256 Immune System 2
Partners
C9ORF72FEM1BFIP200RAB11ARAB8AULK1WDR41
Complex memberships
C9orf72-SMCR8-WDR41

Evidence

Reading pass · 15 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2016 C9orf72 forms a heterodimer with SMCR8, and together with WDR41 constitutes a trimeric complex that associates with the FIP200/ULK1 autophagy initiation complex, supporting a role in autophagy regulation. Co-immunoprecipitation, pulldown, CRISPR/Cas9 knockout mice with phenotypic analysis Acta neuropathologica communications High 27193190
2016 The C9ORF72/SMCR8 complex displays GTPase activity and acts as a GEF for RAB39B; SMCR8 interacts with the ULK1 complex and regulates ULK1 expression and activity to control autophagy initiation; Smcr8 knockout cells show impaired autophagy induction and reduced autophagic flux with abnormal lysosomal enzyme expression. GTPase activity assay (in vitro), GEF assay, Co-IP, Smcr8 knockout mice and cells Science advances High 27617292
2016 C9orf72 robustly interacts with SMCR8 and localizes to lysosomes; lysosomal localization is negatively regulated by amino acid availability. Loss of C9orf72 or SMCR8 causes abnormally swollen lysosomes and impairs mTORC1 signaling responses to amino acid availability. Genome editing (CRISPR KO), Co-IP, live-cell imaging/subcellular fractionation, mTORC1 signaling assays Molecular biology of the cell High 27559131
2020 Cryo-EM structure of the C9ORF72-SMCR8-WDR41 complex at 3.2 Å reveals a dimer of heterotrimers; within the heterotrimer, SMCR8 bridges C9ORF72 and WDR41 without direct C9ORF72-WDR41 contact; WDR41 binds the DENN domain of SMCR8 via its C-terminal helix; Arg147 of SMCR8 (analogous to the arginine finger of FLCN) is critical for GAP activity toward Rab8a and Rab11a, as shown by biochemical mutagenesis. Cryo-EM (3.2 Å), in vitro GTPase GAP assay, mutagenesis (Arg147) Proceedings of the National Academy of Sciences of the United States of America High 32303654
2018 SMCR8 is required for C9ORF72 stability (long isoform of C9ORF72 complexes with and stabilizes SMCR8, enabling interaction with WDR41); Smcr8 knockout mice develop autoimmunity phenotypes, and Smcr8-deficient macrophages exhibit increased lysosomal exocytosis (elevated surface LAMP1 and enhanced secretion of lysosomal components), phenocopying C9orf72 loss-of-function. Quantitative mass spectrometry proteomics, Smcr8 KO mice, LAMP1 surface expression assay, lysosomal secretion assay Genes & development High 29950492
2018 Loss of SMCR8 causes excessive endosomal TLR (TLR3, TLR7, TLR9) signaling due to prolonged ligand-receptor contact; splenomegaly and lymphadenopathy in Smcr8 knockout mice are rescued by triple knockout of endosomal TLRs; Smcr8-deficient macrophages show accumulation of LysoTracker-positive vesicles and delayed phagosome maturation. Genetic epistasis (triple TLR KO rescue), macrophage cytokine assays, LysoTracker staining, phagosome maturation assay Proceedings of the National Academy of Sciences of the United States of America High 30442666
2019 SMCR8 loss leads to a drastic decrease of C9orf72 protein levels; ablation of SMCR8 results in elevated MTORC1 and AKT activation, downregulation of autophagy-lysosome pathway proteins, and increased spine density in neurons. Smcr8 KO mice, western blotting, signaling pathway analysis (pAKT, pS6K) Autophagy Medium 30696333
2019 In c9orf72 or smcr8 mutant macrophages, lysosomal degradation and exocytosis are impaired due to disrupted autolysosome acidification; impaired lysosomal degradation leads to aberrant accumulation of MTOR protein and MTORC1 overactivation; MTORC1 inhibition partially rescues macrophage dysfunction, splenomegaly and lymphadenopathy. Double KO mice, lysosomal pH assay, MTOR protein/signaling analysis, rapamycin rescue experiment Autophagy High 31847700
2019 Smcr8 deficiency impairs axonal transport-dependent autophagy-lysosomal function in motor neurons, causing axonal swellings and motor behavior deficits; Smcr8 haploinsufficiency in C9orf72 KO mice exacerbates axonal degeneration and gain-of-toxicity pathology. Smcr8 KO mice, motor behavior testing, axonal transport assays, autophagy flux assays Human molecular genetics Medium 31625563
2021 CRL2FEM1B E3 ligase recognizes an SMCR8 C-degron (Arg/C-degron) to regulate SMCR8 protein lifetime; crystal structure of FEM1B bound to SMCR8 C-degron peptide was solved. Structural biology (crystal structure), Co-IP, biochemical binding assays Biochemical and biophysical research communications High 33892462
2021 Cryo-EM structure of C9orf72-SMCR8 at 3.8 Å reveals two distinct dimerization interfaces involving an extensive interaction network; homology to FLCN-FNIP2 GAP complex enabled identification of a key active-site residue in SMCR8; a coiled-coil region in the uDENN domain of SMCR8 serves as an interaction platform, and its deletion reduces interaction of the C9orf72-SMCR8 complex with FIP200 upon starvation. Cryo-EM (3.8 Å), deletion mutagenesis, Co-IP (starvation conditions) PLoS biology High 34297726
2020 SMCR8 associates with many components of the ubiquitin-proteasome system and is itself poly-ubiquitinated without obvious degradation; endogenous SMCR8 localizes to cytoplasmic stress granules; SMCR8 protein levels are positively linked to C9orf72 protein levels in vivo. Mass spectrometry interactome, ubiquitination assay, immunofluorescence localization Acta neuropathologica communications Medium 32678027
2023 The C9orf72-SMCR8 complex acts as a GAP for RAB8A to suppress primary ciliogenesis; C9orf72 is the RAB8A-binding subunit and SMCR8 is the GAP catalytic subunit; loss of C9orf72 or SMCR8 leads to elongated primary cilia and increased sensitivity to Hedgehog signaling in multiple tissues. Biochemical GAP assay, cell biology (ciliation measurement), KO mice (brain, kidney, spleen), Hedgehog signaling assay Proceedings of the National Academy of Sciences of the United States of America High 38064514
2024 Confirmed that C9orf72 is the RAB8A-binding subunit and SMCR8 is the GAP catalytic subunit within the C9orf72-SMCR8 complex; RAB8A GAP activity mediates suppression of primary ciliogenesis and Hedgehog signaling. Biochemical analysis, cell biology experiments Autophagy Medium 38293807
2025 The C9orf72/SMCR8 complex regulates lysosomal repair in microglia via a RAB8A-ESCRT mechanism; loss of the complex causes accumulation of GTP-bound (hyperactivated) RAB8A that becomes aberrantly hyperphosphorylated and mislocalizes to non-lysosomal vesicles; defective ESCRT recruitment to damaged lysosomes was observed; GAP activity of the complex is essential for lysosomal repair. C9orf72/SMCR8 KO mice, LLOMe-induced lysosomal damage model, galectin-3 recruitment assay, phospho-RAB8A imaging, ESCRT recruitment assay bioRxivpreprint Medium bio_10.1101_2025.08.22.671707

Source papers

Stage 0 corpus · 17 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2016 The ALS/FTLD associated protein C9orf72 associates with SMCR8 and WDR41 to regulate the autophagy-lysosome pathway. Acta neuropathologica communications 234 27193190
2016 A C9ORF72/SMCR8-containing complex regulates ULK1 and plays a dual role in autophagy. Science advances 188 27617292
2016 C9orf72 binds SMCR8, localizes to lysosomes, and regulates mTORC1 signaling. Molecular biology of the cell 148 27559131
2020 Cryo-EM structure of C9ORF72-SMCR8-WDR41 reveals the role as a GAP for Rab8a and Rab11a. Proceedings of the National Academy of Sciences of the United States of America 60 32303654
2018 The C9orf72-interacting protein Smcr8 is a negative regulator of autoimmunity and lysosomal exocytosis. Genes & development 55 29950492
2019 C9orf72 and smcr8 mutant mice reveal MTORC1 activation due to impaired lysosomal degradation and exocytosis. Autophagy 44 31847700
2018 Excessive endosomal TLR signaling causes inflammatory disease in mice with defective SMCR8-WDR41-C9ORF72 complex function. Proceedings of the National Academy of Sciences of the United States of America 42 30442666
2019 SMCR8 negatively regulates AKT and MTORC1 signaling to modulate lysosome biogenesis and tissue homeostasis. Autophagy 26 30696333
2019 Smcr8 deficiency disrupts axonal transport-dependent lysosomal function and promotes axonal swellings and gain of toxicity in C9ALS/FTD mouse models. Human molecular genetics 16 31625563
2020 C9orf72-associated SMCR8 protein binds in the ubiquitin pathway and with proteins linked with neurological disease. Acta neuropathologica communications 15 32678027
2021 Structural insights into SMCR8 C-degron recognition by FEM1B. Biochemical and biophysical research communications 14 33892462
2020 The C9orf72-SMCR8-WDR41 complex is a GAP for small GTPases. Autophagy 12 32521185
2017 Multifaceted role of SMCR8 as autophagy regulator. Small GTPases 10 28696821
2021 Structure of the human C9orf72-SMCR8 complex reveals a multivalent protein interaction architecture. PLoS biology 8 34297726
2023 ALS-linked C9orf72-SMCR8 complex is a negative regulator of primary ciliogenesis. Proceedings of the National Academy of Sciences of the United States of America 7 38064514
2024 The C9orf72-SMCR8 complex suppresses primary ciliogenesis as a RAB8A GAP. Autophagy 6 38293807
2021 Molecular interactions between C9ORF72 and SMCR8: A local energetic frustration perspective. Biochemical and biophysical research communications 3 34256240